Hematopoiesis is the development and formation of all blood cell types. All peripheral blood cells are derived from pluripotential stem cells. These cells are capable of both self-renewal and sequential commitment to the diverse blood cell lineages. The process of lineage commitment is orchestrated by key transcriptional regulators that have been shown to organize into dynamic multi-protein complexes. These transitory complexes are thought to promote one lineage-specific developmental program at the expense of the others. The long-term objectives of this research are to better understand the molecular genetic strategies that regulate hematopoiesis. The Drosophila model system is ideally suited for studying multiple genetic interactions and was used to show that combinatorial interactions between GATA, Friend of GATA (FOG) and Runx proteins regulate hematopoietic lineage commitment. Specifically, the GATA factor SerpentNC (SrpNC) was shown to act as both a negative and positive regulator of lineage commitment. Negative regulation is dependent upon complex formation between SrpNC and the FOG protein (U-shaped). Positive regulation results from an interaction between SrpNC and the Runx-related p-otein (Lozenge), which also down-regulates the repressor component U-shaped. Based on these results, the specific hypothesis behind this proposal is that SrpNC acts as a cross-regulatory switch during blood cell lineage commitment. The following specific aims are designed to investigate the molecular genetic mechanisms that direct the SrpNC cross-regulatory network during lineage commitment: 1) Test if the SrpNC:Lozenge complex directly down-regulates u-shaped gene expression and recruits U-shaped as an auto-repressor; 2) Identify activators of u-shaped hematopoietic gene expression; 3) Test if lozenge gene expression is down-regulated by the SrpNC:U-shaped complex and if this pathway functions during larval hematopoiesis The results from these studies will increase our knowledge of lineage commitment and the etiology of blood disorder.